T-318
3
−
10
3.3.19 Defrost Initiation
Initiation of defrost is dependent on the state of the De-
frost Temperature Thermostat (DTT). The functionality
of the DTT is controlled by the system software, based
on the temperature reading of the Defrost Termination
Sensor (DTS). In the case of a failed DTS, the RTS may
be used to determine the state of the DTT.
Defrost cannot be initiated when the DTT is considered
OPEN, because an open DTT indicates that the temper-
ature is too high for ice to build up. Defrost can only oc-
cur when the temperature sensed by the DTS is low
enough to CLOSE the DTT.
If the DTT is closed, the unit is in Perishable or Frozen
Mode, and the Controlled Atmosphere Vent is closed (if
applicable), then defrost can be initiated when any one
of the following additional conditions become true:
1. Manual defrost is initiated by the operator. The
Manual Defrost Switch (MDS), located on the front of
the unit, is a momentary toggle switch that must be
held closed until defrost is initiated. The MDS is ig-
nored during Pretrip, and the DTT must be con-
sidered CLOSED for MDS activation to work.
Manual defrost can also be initiated by pressing the
MANUAL DEFROST/INTERVAL key for greater
than 5 seconds, or pressing the PRE TRIP key and
the ALT MODE key for greater than 5 seconds. If the
two key method is used the display may show “P” or
“Dc” while waiting for defrost to initiate.
2. The Defrost Interval Timer reaches or exceeds the
Defrost Interval.
3. During Pretrip, defrost may occur during tests P-8,
and P-10. Defrost is forced during Pretrip test P-9.
4. Temperature Probe Diagnostics logic determines
that a Probe Check is necessary based on temperat-
ure values currently reported by the supply and re-
turn probes.
5. An Initiate Defrost command is sent via communica-
tions.
6. The controller Demand Defrost configuration vari-
able (CnF40) is set to “In” and the unit has been in
pull down operation for over 2.5 hours without reach-
ing set point.
7. The microprocessor can determine if defrost is re-
quired by calculating the difference between return
air temperature and supply air temperature (Delta
T). If the temperature difference between return air
and supply air is too great, it indicates reduced air-
flow over the evaporator coil, and a defrost cycle
may be required:
a. In Perishable Pull Down - Defrost will be initiated if
Delta T increases to greater than 12
C, DTT is
closed, and 90 minutes of compressor run time
have been recorded.
b. In Frozen Mode - Defrost will be initiated if Delta T
increases to greater than 16
C, DTT is closed, and
90 minutes of compressor run time have been re-
corded.
c. In Perishable Steady State – A baseline Delta T is
used to determine when defrost should be activ-
ated. The baseline is recorded after completion of
an initial defrost cycle. In order to record a baseline
Delta T, the unit must be cooling, and the evaporat-
or fans and heaters must remain in a stable state
for a period of five minutes. Defrost will then be initi-
ated if Delta T increases to greater than 4
C above
the baseline, the DTT is closed, and 90 minutes of
compressor run time have been recorded.
NOTE
During defrost, the Defrost Relay is energized,
the orange DEFROST light is illuminated, and
the orange HEAT light is illuminated.
Defrost may be initiated any time the defrost temperat-
ure sensor reading falls below the controller DTT set
point. Defrost will terminate when the defrost temperat-
ure sensor reading rises above the DTT set point. The
DTT is not a physical component. It is a controller setting
that acts as a thermostat, “closing” (allowing defrost)
when the defrost temperature sensor reading is below
the set point and “opening” (terminating or preventing
defrost) when the sensor temperature reading is above
set point. When the unit is operating in bulb mode (refer
to paragraph 3.3.7), special settings may be applicable.
If the controller is programmed with the Lower DTT set-
ting option, the defrost termination thermostat set point
may be configured to the default of 25.6
C (78
F) or
lowered to 18
C (64
F). When a request for defrost is
made through the manual defrost switch, communica-
tions or probe check the unit will enter defrost if the DTT
reading is at or below the DTT setting. Defrost will ter-
minate when the DTS reading rises above the DTT set-
ting. When a request for defrost is made with the defrost
interval timer or by demand defrost, the defrost temper-
ature setting must be below 10
C (50
F).
When defrost is initiated, the controller closes the EEV,
opens contacts TC, TN and TE (or TV) to de-energize
the compressor, condenser fan and evaporator fans.
The white COOL light is also de-energized. The control-
ler then closes contacts TH to supply power to the heat-
ers, and the orange DEFROST light is illuminated.
When the DTS reading rises to the DTT setting, the
de-icing operation is terminated.
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE-ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
SIGNAL TO
CONTROLLER
Figure 3-10 Defrost